Two teenage cousins have died, two children are seriously hurt and a father is injured following a crash involving off-road motorbikes in Melbourne’s west.

The crash happened on Beattys Road at Grangefields, about half an hour from Melbourne’s CBD, at 6.35pm on Sunday.

Eighteen-year-old Zena Houli, of Altona Meadows, and her 17-year-old Hoppers Crossing cousin Ali El Houli, of Hoppers Crossing, collided head on while riding on a sealed public road, Victoria Police Detective Senior Sergeant Paul Lineham said.

He said the crash happened near a private property where about 50 people had gathered and they rushed to help.

Omar El Houli, who is the teenagers’ uncle, told the Nine Network he was the first to discover the teenagers after the horrific crash.

The woman died at the scene. The boy was taken to hospital where he later died.

Two boys, aged six and 11, were also injured and taken to a local hospital by family. Investigators do not know if they were on the back of the dirt bikes or watching.

“Unfortunately, we haven’t received much in the way of co-operation with the family so we’re not too sure as to how they received those injuries,” Senior Sergeant Lineham said on Monday.

“It’s incredibly hard for police to do their job when people won’t provide the details.”

A 43-year-old Altona Meadows man, who is the father of Ms Houli, was seriously injured after his quad bike hit a road sign. It is believed he was responding to the first crash.

“It appears the father of the deceased has heard the collision and has hopped onto a quad bike himself and has ridden to the scene,” Sergeant Lineham said.

“He was probably 20 metres from the scene and he also lost control of his bike and sustained non-life-threatening injuries.”

Senior Sergeant Lineham said the bikes were not registered and were unlicensed.

“They’re not fit for purpose on the road anyway and they shouldn’t have been on it in the first place,” he said.

Victoria’s road toll has risen to 202, up from 162 this time last year.

Anyone who witnessed the incident, has footage or information about what happened is urged to contact police.

-with AAP